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    Stress Corrosion Cracking on Steam

    Turbine Rotor Grooves: Experiences and

    Countermeasures from EGAT Power

    Plants

    Kobchai Wasuthalainan, Kanit Nangkala,

    and Santhiti Chantha-Uthai

    Electricity Generating Authority of

    Thailand (EGAT), Thailand

    Abstract

    Although Stress Corrosion Cracking (SCC) can occur in many locations of steam

    turbine, most of them initiate at LP disc rim, rotor groove or blade attachment area.

    Usually power plants operating more than 15 years are susceptible to this failure

    mechanism. If SCC happens, especially in rotor groove, it has a major influence on steam

    turbine life. Because of the complexity of crack growth behavior, it is difficult to estimate

    the remaining life of the rotor with cracks found. In some cases short term remedies are

    urgently needed in order to return the unit back in operation before long term actions can

    be decided. For our steam turbines in Electricity Generating Authority of Thailand

    (EGAT) that faced such problem, we had many experiences and countermeasures both

    from Original Equipment Manufacturers (OEMs) and in-house learning by doing. Several

    corrective actions, for example blades cutting (partial or entire row) with or without

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    baffle plate installed, skin machining with retouching included, removal of all blades

    from row, continuing in operation with periodical inspection, or even retrofitting will

    yield different results and different costs-benefits. Maximum crack depth, allowed outage

    duration, spare parts and repair cost are the key factors for deciding on which suitable

    actions to be taken. Countermeasures from EGAT experiences may be used as a guideline

    for other units which encounter similar problem.

    1. Introduction of Stress Corrosion Cracking Mechanism

    In LP turbine section, cracking on rotor due to SCC has been most prevalent in

    fir-tree or inverted fir-tree groove designs but it also has been noted in other designs such

    as finger type design.[3]Cracks initiate from surface especially around the notches of the

    grooves and propagate across the steeples. Failure mechanism is a function of stress

    intensity, rotor material, and steam environment so that the probability of occurrence is

    higher in the longer last stage blades when the moisture of condensation begins to form.

    Figure 1 shows an example of SCC found on LP rotor groove

    Figure 1Example of Stress Corrosion Cracking on LP rotor groove

    (left) crack and pitting corrosion at LP rotor groove

    (right) microstructure of crack examined by replication

    Corrosion induced cracks are brittle, usually branching, and may be either trans-

    granular or inter-granular depending upon the material composition, stress levels, and

    corrosive environment.[3]

    However for EGAT steam turbine rotors that are subject to

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    some heat treatment processes, it can be seen from figure 1 that SCC exhibits multi-

    branched inter-granular crack pattern in martensitic steel.

    Crack propagation rate from SCC can be divided into 3 regions as a function of

    applied stress intensity factor (KI) which are dependent, constant, and approaching final

    failure.[3]

    In the first phase, SCC can only propagate when stress intensity at rotor groove

    exceeds the threshold of the crack growth (KISCC). When KI

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    Figure 2Replication test after several indications on rotor grooves were found

    Once SCC was found, actual crack dimensions had been recordedfor reference.In addition, the most two crucial things to be determined are critical crack depth and

    crack growth rate. The critical crack depth (acr) can be calculated by Finite Element

    Analysis (FEA) software where acr is the minimum crack depth that the applied stressintensity factor (KI) exceeds the material toughness (KIC).

    [1]Note that KIis the function

    of assumed crack geometry or flaw shape and KICdepends on material properties.

    Since plateau crack growth rate during stable or constant growth period was

    purposed in generic form as

    21 3ln y

    CdaC C

    dt T

    = +

    [1]

    where

    da

    dt= the crack growth rate

    1C , 2C , 3C = material constants

    T= the operating temperature of the disc

    y= the room temperature yield strength of the disc

    Hence, the remaining life of turbine rotor from static stress during operation can be

    computed from

    cr ir

    a at

    da

    dt

    =

    [1]

    where

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    da

    dt= the crack growth rate

    rt = the remaining life

    cra = the critical crack depth

    ia = the detected crack depth

    In general, there is not only static stress during normal operating conditions but

    also dynamic stress which eventually be generated from cyclic operation. Frequent

    numbers of unit start/stop can stimulate dynamic loads when temperature gradients in

    steam turbine rotor increase. This behavior acts as a driving force to build up the defects.

    If the cyclic stress intensity (K) is greater than the threshold value (Kth), fatigue

    cracking will combine with SCC. Suitable model for crack growth prediction should be

    able to cope with HCF or LCF in the latter case. Dynamic crack growth can be expressed

    as

    ( )nda

    C KdN

    = [2]

    where

    da

    dN

    = the crack growth rate

    C, n = empirical constants

    K = the cyclic stress intensity factor

    However the method of remaining life evaluation for SCC damage mechanism is

    quite like a step by step procedure. Practically, there are many uncertainties or errors

    occur, for example flaw sizing, Non-Destructive Test (NDT) methods, scatter data on KIC

    from laboratory, or error from calculation. Deterministic approach may result in

    pessimistic or optimistic outcomes. Therefore probabilistic analysis that considers input

    probability functions by using Monte Carlo Simulation may present a more realistic

    outcome. For this reason, determination of remaining life is not a simple task so EGAT

    occasionally consults OEMs or specialists in order to confirm the evaluation results for

    making further corrective actions.

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    3. EGATs Experiences and Countermeasures

    From our EGATs steam turbine fleet maintenance historical records, rotor

    groove inspection program has been first carried out since November 1997 for detecting

    SCC. There are 12 out of 46 units which indications were found including LP steam

    turbine rotors from EGAT subsidiary companies such as Electricity Generating Company

    (EGCO) and Ratchaburi Electricity Generating Company (RGCO). However, we have

    not counted for South Bangkok Combined Cycle Block 3 (SB-C30), Bankpakong

    Combined Cycle Block 5 (BPK-C50), and Chana Combined Cycle Block 1 (CNP-C10)

    from this statistical point of view since these power plants are in commissioning or in

    warranty periods. According to our SCC rotor groove damage records, all of them were

    axial-entry fir tree grooves and cracks mostly occurred in the last three stages of LP

    turbines which hereinafter called L-0, L-1, and L-2 respectively. The size and shape of

    cracking varied unit by unit. The maximum crack depth and length depend on aging of

    steam turbine, operating conditions, and different in design. Although the location of

    crack could be initiated from any steeple as shown in figure 3, the crack was most

    frequently found at steeple 1 and it was usually the critical location because the critical

    crack depths (acr) is less than any other steeples in nature.

    Figure 3Schematic of axial-entry fir tree groove design in LP rotor with typical cracking locations shown

    Table 1 shows the summary of SCC damages found on 11 of 17 units that had

    performed life assessments. All of them have been operated for more than 15 years.

    Indications were found at the stage of L-0, L-1, and L-2 in total amount of 8, 6, and 2

    Steeple 1

    Steeple 2

    Steeple 3

    End Face

    Rotor Groove

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    rotors respectively. In addition, we observed that crack could propagate both from convex

    or concave side and may originate from the end face or in the middle of the groove.

    Table 1SCC damage records on LP rotor grooves

    SCC IndicationsNo. Plants

    COD

    (year)

    Inspection

    (year) L-0 L-1 L-2

    Sampling

    (/stage)Countermeasures

    1 NB-T1* 1961 2003 No No No 1 groups No Actions

    2 SB-T1** 1970 1998 No No No 2 groups No Actions

    3 SB-T2** 19711997

    2000

    Yes

    Yes

    Yes

    Yes

    No

    No

    2 groups

    100%

    Grinding

    Grinding

    4 SB-T3 1974

    1997

    2000

    2001

    2006

    Yes

    Yes

    Yes

    N/A

    N/A

    No

    N/A

    N/A

    N/A

    No

    N/A

    Yes

    1 group

    100%

    100%

    at end face

    Grinding

    Grinding

    Re-machining (drop steeple)

    Blade Cutting

    (with Baffle Plate installed)

    5 SB-T4 1975

    2001

    2005

    Yes

    N/A

    No

    N/A

    No

    Yes

    100%

    at end face

    Re-machining (drop steeple)

    Blade Cutting

    (with Baffle Plate installed)

    6 SB-T5 1977

    1999

    2002

    2007

    No

    Yes

    N/A

    No

    N/A

    N/A

    No

    N/A

    No

    2 groups

    100%

    at end face

    No Actions

    Re-machining (drop steeple)

    Welding Repair (partial)

    No Actions

    7 MM-T1*** 1977 1998 N/A Yes N/A 5 grooves Grinding

    8 MM-T3*** 1978 1999 Yes N/A N/A 100% Grinding

    9 MM-T4 1984 2002 No No No 2 groups No Actions

    10 MM-T5 1985 2008 No No No 2 groups No Actions

    11 MM-T6 1985 2005 No No No 2 groups No Actions

    12 MM-T7 1985 2007 No No No 2 groups No Actions

    13 MM-T8 1989

    2004

    2006

    2008

    Yes

    Yes

    No

    N/A

    N/A

    Yes

    N/A

    N/A

    N/A

    100%

    12 grooves

    17 grooves

    Grinding

    Grinding

    LP Turbine Retrofit

    14 MM-T9 1990

    2006

    2007

    Yes

    N/A

    Yes

    N/A

    Yes

    N/A

    100%

    N/A

    Blade Removal

    (without Baffle Plate installed)

    LP Turbine Retrofit

    15 MM-10 19912006

    2009

    Yes

    Yes

    Yes

    Yes

    No

    N/A

    100%

    10 grooves

    Grinding

    LP Turbine Retrofit

    16 BPK-T1 19832003

    2005

    No

    N/A

    Yes

    N/A

    No

    N/A

    1 groupat end face

    N/A

    GrindingRemaining Life Evaluation

    LP Turbine Retrofit

    17 BPK-T2 1983

    2001

    2003

    2005

    2006

    No

    N/A

    N/A

    N/A

    Yes

    Yes

    Yes

    N/A

    No

    N/A

    N/A

    N/A

    1 group

    at end face

    at end face

    at end face

    N/A

    Grinding

    Remaining Life Evaluation

    Remaining Life Evaluation

    Remaining Life Evaluation

    LP Turbine Retrofit

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    Remarks:

    - Units with mark *, **, and ***have been disconnected from the grid in year 2002, 2008, and 2000respectively.

    -

    N/A means no information or not available during that time.-NB-T1 to NB-T3, SB-T1 to SB-T2, SB-T3 to SB-T5, MM-T1 to MM-T3, MM-T4 to MM-T7,

    MM-T8 to MM-T13, and BPK-T1 to BPK-T2 are of the same type group by group.

    - The material of all rotors is NiCrMoV steel forging.- The other 4 power plants, not listed in the table above, had been performed condition

    assessments which are RY-C10 to RY-C40. No indications were reported at that time but notice

    that only magnetic particle testing had been done around the finger type grooves on some

    selected stages.

    When SCC is detected, repairs or corrective actions must be considered carefully.

    For short terms, some remedies should have been done in order to resume the unit back

    into operation. These actions may help to restore the operator confidence and reduce

    operating risks. Furthermore, these can give us some extended time before finding the

    long term corrective actions. From our experiences, if the critical crack depth and crack

    growth rate can be accurately calculated and objective constraints from power plant are

    cleared then we may select and implement a suitable long term strategy for unit in both

    economical and technical aspects. Examples of short term countermeasures are runninguntil next outage with or without load limitation, crack grinding, blade cutting or removal

    which can be done with or without baffle plate installation. The others are long term

    actions that can be welding repair, dropped steeple machining, or rotor replacement.

    These actions may be conducted with an improve design such as material upgrading, re-

    blading with titanium[3]

    or modification with large blade root and groove to minimize the

    centrifugal stress[7]. This section will describe each countermeasure that we had faced in

    the cost-benefit way. The items that EGAT has encountered are as follow.

    - Crack grindingOne common countermeasure to cracks found on EGATs LP rotor grooves is

    grinding technique. After NDT confirmed cracks discovered, grinding with small

    grinding machine (Figure 4) would be performed by skilled technicians. The first

    grinding will be limited to about 0.1 mm deep before another NDT to check for crack

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    remaining. If cracks still exist, another 0.1 deep grinding and NDT will be repeated until

    there is no crack remained. The final depth and length of all ground cracks are measured

    by using silicone replication, and recorded along with cracks location in the inspection

    sheet for further consideration. The whole process of crack grinding normally takes about

    2 days per turbine stage. However the duration may vary depending on numbers and

    depth of cracks discovered.

    Figure 4Steps of crack grinding

    (Left) Crack found on LP rotor grooves

    (Middle) Grinding-off crack found on LP rotor groove with small milling

    (Right) 0.5 mm depth of ground-off crack

    (Bottom-Left) Silicone replication of ground crack for measuring final depth & length

    (Bottom-Right) Inspection sheet recorded all ground cracks found

    Crack grinding is common solution to every crack found on LP rotor grooves

    discovered at EGAT power plant. This countermeasure can be considered as short to

    medium term corrective actions, depending on the maximum crack depth found. If the

    deepest ground crack depth found on rotor grooves is not over the critical depth allowed,

    rotor blades can be reinstalled into grooves and resumed normal operation. However, if

    final crack depth exceeding critical depth allowed, that particular groove is considered

    CONCAVE CONVEX

    INLETINLET OUTLET

    INSERTING GROOVE NO.42

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    damaged with less load carrying capability. Blade cutting or removals are, therefore,

    inevitable solutions.

    For every EGAT power plant encounter cracks on LP rotor grooves, crack grinding

    technique is chosen to be the first countermeasure implemented. These happened to all

    five units of South Bangkok thermal plants (SB-T1 to 5), five units of Maemoh thermal

    plants (MM-T1, 2 and MM-T8 to 10) and two units of Bangpakong thermal plants (BPK-

    T1 and 2) where cracks were discovered during the life assessment of steam turbine

    which normally applied in their 15-25 years of services. All cracks were discovered when

    selected LP blade groups were removed for thorough magnetic particle inspection at

    blade-rotor assertion area. In 2004, during life assessment inspection of MM-T8, we

    found many cracks on L-0 stages with final ground-off depth range between 0.85 to 1.0

    mm. Since the depth did not exceed the critical crack depth allowed for this type of

    groove design (maximum crack depth is 4.0 mm), all L-0 blades were reinstalled and

    normal operation was resumed. After one year in service, the unit was down for safety

    reason, for another L-0 grooves inspection to investigate on the ground-off crack growth.

    Although several new cracks at L-0 grooves were found, no ground-off cracks

    propagated. Usually the final ground cracks in LP rotor of EGAT are found to be less

    than 2 mm deep, grinding technique therefore can extend life of LP rotor. MM-T10 can

    continue to operate for up to 3 years. This life extension can be used for decision makingon long term solution.

    - Running until next outageRunning until next outage could be a temporary solution that may have

    economical advantages if continuous operation is the first necessity. Remaining life

    assessment is very important thing before choosing this action. Maximum crack depth

    and crack growth rate with some degree of safety concern are the keys because period of

    plant operation is limited by these two factors. However such evaluation usually takes

    time since calculation method consists of several steps as mentioned earlier above in

    section 2.

    In case of EGAT power plants, this short term corrective action was selected for

    Bangpakong thermal power plant unit 1 and 2 (BPK-T1, 2). Local stress analysis had

    been done by OEM at the same time SCC was found on L-1 steeples of BPK-T2 rotor in

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    2001. The result shows that this groove type has relatively high local stress at the inlet

    and outlet (see figure 5).

    Figure 5Relative stress distributions on L-1 groove (BPK-T1, 2)

    The FEA coincides with actual inspection results both BPK-T1 and 2 performed during

    2003 and 2001 respectively. Firstly, remaining life of 2 LP rotors of BPK-T2 were

    estimated at around 18,500 hours. Because maximum crack depth will occur at the end

    face of rotor groove so we can reexamine the crack depth and crack growth rate without

    removing L-1 blades off. The rotor end face inspection (magnetic particle test) can be

    done every to 2 years by removing cross over pipes and opening upper LP half casings

    without lifting up the rotors. Outage schedule can be accomplished within 25 days. So

    running until next Minor Inspection (MI) was preferred to Bangpakong power plants

    steam turbines. Finally at the end, these failed rotors were replaced and upgraded by

    retrofitting LP turbines in year 2005 and 2006 respectively.

    - Blade cutting or removal (with or without Baffle Plate installation)In the case of ground-off crack depth approaching maximum depth allowed and

    may not last until the next outage, blade cutting or removals become suitable solutions

    for short term remedies. These techniques were applied to several of EGAT power plants

    ranging from a small single cylinder of 30 MW Suratthani thermal plant to large LP

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    rotors of multiple cylinder 300 MW thermal plants (Maemoh and South Bangkok). There

    are varieties of solutions taken by EGAT concerning blade cutting or removal. The

    decisions were made based on degrees of damage and outage time available at each plant.

    EGAT has experienced partial blades removal from one stage (South Bangkok), removal

    of all moving and stationary blades from 2 opposing L-2 stages with baffle plates

    installed (South Bangkok), and all blades removals from 2 opposing L-1 without baffle

    plates installed (Maemoh).

    In 2004, South Bangkok thermal plant unit 4 (SB-T4) encountered SCC problem

    at L-2 stage (gen-side) grooves. After grinding, 6 of 198 grooves had final depth

    exceeding maximum depth allowed. New rotor replacement was not appropriated due to

    shutdown time constraint. In order to resume the plant back in service, full-row of

    moving and stationary blades were removed both side from double flow LP turbine and

    replaced with baffle plates in order to maintain the same pressure drop inside and

    minimize effect to nearby stages. After complete installation, operation resumed with

    limited load of 90%. Estimated time frame for blade removal and baffle plate installation

    is 2 weeks excluding delivery period of baffle plates.

    Figure 6Illustration of baffle plate and installation in place of stationary blade of LP inner casing at SB-T4

    In 2006, Maemoh thermal plant unit 9 (MM-T9) encountered SCC on L-1

    grooves. After grinding, numbers of grooves had ground crack depth over 10 mm, which

    exceeded allowable depth. Unfortunately, blade removal and baffle plates installation

    were not suitable to this case, because of the downtime constraint. To resume the plant in

    service as soon as possible, EGAT had considered the moving blades removal with

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    stationary blades remained. Although this action could cause changes in pressure drop

    across LP stages, especially to the adjacent stages of L-0 and L-2, limiting operation to

    75% load was chosen to mitigate this effect. The unit was operated for 5 months with

    normal conditions before the new LP rotor arrived for replacement. Estimated time frame

    for L-1 blade removal from both sides of LP turbine is 2 days.

    Figure 7Illustrated the full-row blade removal of L-1 stage without installation of baffle plate

    - Dropped steeple machining (with Flow Guide modification plus shot peening)Re-machining groove by precise lathe machine with special cutting tools is the

    one of mechanical repairs. It can be classified into 2 subcategories which are skin cutting

    or dropped steeple machining. Skin cutting will work only for shallow crack. This

    method is usually followed by surface polishing and shot-peening to generate residual

    compressive stress.[3]

    Dropped steeple machining on the other hand is the machining to

    the new pitch circle downward to the centerline of turbine rotor. Both have the same

    disadvantages that are moderately high cost and unfavorable extended outage. Figure 8

    illustrates the idea of dropped notch steeple or undercutting machining. However, groove

    machining with enlarge radii or steeple drop machining may be inappropriate in some

    cases, BPK-T1 and 2 for example, 3rd

    steeple crack still exists in groove as shown in

    figure 8 (left). Limitation of re-machining is the geometry of groove itself.

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    Figure 8example of LP rotor groove modification

    (left) groove machining with enlarge radii

    (right) steeple drop machining

    South Bangkok thermal power plant unit 3 to 5 had been tired dropped steeple

    machining in year 2001, 2001, and 2002 respectively. The rotors were shipped to OEM

    factory for machining. Some modification works on L-0 rotating blades, flow guides, and

    gland cones were required since the dimension of rotor groove had been changed. For L-

    0 rotating blades, blades had been removed and the platforms were machined a little bit

    thinner than original shape as shown in figure 9. Lining plates have been attached into

    flow guides and gland cones have been grinded as shown in figure 10. Estimated time

    frame for corrective is around 9 months including transportation.

    Figure 9L-0 rotating blade platform grinding

    Existing Shape

    Shape after Re-machining Cut-off by Lathe

    Cut-off by Special CutterCrack Remains

    Existing GrooveRemove Crack

    TENON

    CONCAVECONVEX

    PLATFORM

    LEADING EDGE

    GRINDING

    ROOT SERRATIONS

    ROOT

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    Figure 10Flow guide and gland cone modification (detail A and B)

    From EGATs operating experience, this countermeasure can be considered as a medium

    term corrective action when the objective for power generation ranges from 5 to 10 years.

    The reason behind is that there is high probability that SCC will developed in other stages

    years after. If SCC reoccurs again somewhere on LP rotor then it would be troublesome

    and SB-T3 and 4 have been suffered from this case. It can be seen from Table 1 that SCC

    was found again but at L-2 stage instead. Plants must be connected to the dispatch system

    until 2010 so blades cutting with pressure plates installed were taken as short term

    remedy. Decision making, based on cost minimization, for SB-T3 and 4 in 2006 and

    2005 can keep units running until it will be removed from the grid but of course with

    higher heat rate or lower electricity output.

    - Welding repair (partially along with drop steeple machining)One possible method to eliminate SCC from rotor grooves is welding. Filler metal

    of same composition as rotor material or highly SCC resistant material can build up the

    new groove partially or entire row. After machining or cutting damaged grooves, welding

    is commonly employed layer by layer together with pre-heat control. The direction of

    finishing layer is different from others to make the shape of welding better. Re-machining

    of grooves proceeds by side-entry machine. Penetrant test, magnetic particle test, and

    ultrasonic test shall be executed for checking porosity or crack. Post weld heat treatment

    L-0 Stationary Blade

    Flow Guide

    L-0 Rotating Blade

    Gland Cone

    B

    A

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    must be carefully controlled. Moreover, hardness test and replication test should be

    confirmed after post weld heat treatment.

    Figure 11Schematic of welding repair steps

    (left) Cutting grooves after blade removal

    (middle) Hammering should be proceeded to prevent undercut

    (right) Repair welding

    EGAT has one LP rotor welding repair case which is South Bangkok thermal

    power plant unit 5. Since only dropped steeple machining could not get rid of some

    cracks on 3rd

    steeple of rotor then partial welding repair had been employed in additional.

    Figure 12 shows photographs of L-0 grooves during building up and machining

    processes. Note that repair job was done at OEM workshop in 2002.

    Figure 12building up metal and re-machining on L-0 grooves (SB-T5)

    (left) Penetrant test on build up metal after finished welding repair

    (right) L-0 re-machining

    - Rotor replacementLP turbine replacement can be the final solution for SCC problem but with very

    high investment cost and long time to implement usually not less than 2 years. Rotor

    replacement can be with original prototype or upgraded design. The latter could achieve

    benefits from heat rate improvement or SCC resistance. Retrofit or modernization gains

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    more advantages from upgrading rotor material or applying state of the art technology.

    Technical and commercial point of view should be taken into account when retrofitting.

    Economic feasibility should be studied for helping decision making. The acceptance

    criteria, for example performance guarantee or SCC warranty should be written in the

    contract for vendor. Furthermore nearby component interface compatibility such as

    Generator, HP/IP turbine, or extraction piping should be examined during execution

    phase of LP turbine retrofit. This action is considered as long term countermeasure.

    Bangpakong thermal power plant unit 1 and 2 had LP turbine retrofits in year

    2005 and 2006 or 2 and 5 years from first SCC find. Maemoh thermal power plant unit 8

    to 10 had LP turbine retrofits between 2007 and 2009, approximately 1 to 4 years since

    SCC had been detected. Pre and post performance tests under international standard test

    code were performed to calculate the increasing MW output. The results show

    satisfactory efficiency improvement both Bangpakong and Maemoh. Hence economical

    value can be met since BPK-T1 and 2 have a plan to extend operating period for another

    10 years. MM-T8 to 10 have a plan to extend period for not less than 15 years.

    4. Conclusion

    No unique countermeasure for solving SCC problem on LP rotor groove istypically applied because of inspection, operation, and economic constraints. There is

    also significant variation among various manufacturers. The influence of the environment

    which is generated during operation and startup-shutdown varied from rotor to rotor and

    from stage to stage on a given LP rotor.[3]

    Major root causes are the combination of

    susceptible material, steam environment, and localize stress concentration which may be

    difficult to determine. However, detection of SCC indications on LP rotor can be done for

    units around 15 years in operation. Despite the fact that SCC problem is reduced through

    using upgrade material, applying compressive stresses, or minimizing stress

    concentration in the blade attachment areas in most of modern LP rotors manufactured

    today, severe environment composition may influence on SCC initiation with unknown

    period of times.

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    The maximum crack depth, allowed outage duration, spare parts and repair cost

    are the key factors for deciding on which suitable actions to be taken. Appropriate actions

    shall take cost-benefit into account. Inspection programs and countermeasures from

    EGAT experiences may be used as a guideline for other units which encounter similar

    problem.

    Acknowledgements

    The authors would like to thank POWER-GEN Asia 2009 advisory board

    committee for selecting this paper abstract and giving the speaker opportunity to present

    our work at the conference. We would like to acknowledge several input materials from

    the following engineers, our colleagues in EGAT, who are Kanit Nangkala, Parichart

    Suttiprasit, Kittithat Manochai, Niphon Punopakorn, Supawat Kladthong, Marut

    Pitayachaval, and Saeree Ruangsawat. Each fulfills in the areas where more detail

    information is required. We also wish to express our gratitude to Chattiya

    Chadhanayingyong for correct proof.

    Special thanks belong to Werasak Hormkrajai, Material Testing Section Manager

    from Mechanical Testing Department who had advised us by using his lot of experiences

    in metallurgical field. Special thanks also go to Santhiti Chantha-Uthai, our SteamTurbine Engineering Section Manager, for reviewing this manuscript. Both of their

    comments were invaluable.

    Finally, benefits from this paper are dedicated to our parents. Any mistakes or

    errors are from us.

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